Argument A1.2.2.3.3.4.4.3.1.2.1 Analysis indicates that the control system's failure rate is acceptable

• Supports Claim: C1.2.2.3.3.4.4.3.1.2 The (intelligent) control system has a tolerable failure rate

Child nodes:

• Has evidence Evidence: E1.2.2.3.3.4.4.3.1.2.1.2 Review of bespoke software and firmware design, implementation and test procedures and outputs
• Has evidence Evidence: E1.2.2.3.3.4.4.3.1.2.1.1 Parts count hardware reliability calculation
• Has evidence Evidence: E1.2.2.3.3.4.4.3.1.2.1.3 COTS equipment failure rate analysis report
• Has evidence Evidence: E1.2.2.3.3.4.1.2.1 Task analysis report
• Has evidence Evidence: E1.2.2.3.3.4.1.2.2 Human factors substantiation

A parts count hardware reliability calculation has been performed.

The bespoke software and firmware has been designed, implemented and tested to good commercial standards.

COTS equipment failure rates have been acquired and analysed.

The total of the above indicates a failure rate < 2 pdfy

Whilst a failure rate for the control system of <1 pdfy is RGP and recognised in IEC 61508, and the analysis performed above deviates from this and cannot give satisfactory confidence that this is the case. However:

• An analysis of the SIF has shown that it can tolerate up to 10 challenges per year with the documented maintainence and proof test schedule.
• The task analysis and Human Factors substantiation have shown that a challenge to the SIF resulting in operator intervention is tolerable once per 24 h period

and therefore deemed to be acceptable.